Primality proof for n = 20187911:
<p>Take b = 2.
<p>b^(n-1) mod n = 1.
<p><a href=42953.html>42953 is prime.</a>
<br>b^((n-1)/42953)-1 mod n = 4757465, which is a unit, inverse 10221519.
<p>(42953) divides n-1.
<p>(42953)^2 > n.
<p>n is prime by Pocklington's theorem.